This invention pertains to the apparatus and method for calibrating the thermal emissivity of semiconductor wafers, and more particularly to a sample-calibrating susceptor and method that holds a sample wafer while measuring its surface temperature by conduction and radiation sensors. Once correlated, these measurements are the basis for correcting the radiative measurements of surface temperature of similar wafers in a production-processing environment.
Contemporary flash-annealing processes on semiconductor wafers rely upon high-intensity sources of non-coherent light to irradiate the surface of the wafer and thereby rapidly heat the wafer from the surface, or surfaces. This process may take place within a reaction tube of quartz which confines the wafer within a controlled atmosphere of selected gases and which transmits the high-intensity light flux from the external light sources, through the walls of the quartz tube, to the surface(s) of the wafer. In the past, it has been difficult to determine with requisite degrees of accuracy the actual temperature at the surface of the wafer that is rapidly heated in this manner. The problem of measuring surface-temperature of a wafer is further complicated by the myriad different circuit patterns which may be formed using layers of diverse materials assembled in complex arrays on the surface that is to be irradiated. The absorptive properties of the semiconductor material that forms the wafer, as well as the surface emissivity of the wafer, can be significantly altered by such circuit patterns on the surface. This contributes to substantial temperature errors in open-loop heating systems which supply a given level of radiation for a timed interval, and also contribute to substantial errors when surface temperature is determined by optically sensing the thermally-emitted radiation.
In accordance with the present invention, an improved susceptor supports a sample wafer within a processing chamber in exactly the same conditions therein in which subsequent wafers are to be processed. The susceptor includes a graphite support element for a sample wafer with a temperature-sensing element such as a thermocouple positioned therein to be closely proximate the center of the sample wafer that is held to the susceptor by vacuum. The susceptor has approximately the same diameter as the wafer and a thickness that is typically not greater than about six times the thickness dimension of a sample wafer. This assures reasonably prompt thermal equilibration of susceptor and sample wafer under conditions within a processing chamber of irradiation from high-intensity non-coherent light sources that are positioned outside the chamber. Thus, a thermal sensor within the susceptor in high thermally conductive contact with the sample wafer accurately measures the actual temperature of the wafer (and therefore, of its surface) under conditions of spacing from the walls of the chamber, atmosphere within the chamber, circuit arrays on the surface, and the like, that will be similar for all subsequent similar wafers to be processed within the chamber.
At the same time, the surface temperature of the sample wafer is detected, for example, by an optical pyrometer which senses radiation from the sample wafer (and from the walls of the processing chamber, and from other radiative sources within the field of view of the pyrometer). The measurement of temperature by optical pyrometry can therefore be correlated with the measurement by thermocouple of the actual temperature of the sample wafer in order to correct the pyrometer reading. Thereafter, subsequent radiative temperature measurements of similar wafers within the processing chamber under substantially identical conditions to those employed in thermally measuring the temperature of the sample wafer greatly improves the speed and accuracy of the measurements of temperatures of similar wafers being processed within chamber.